Osmosis. Potatoes. You know the one...

I'm giving my friend a hand with her AS coursework... and its the good old osmosis with the mass change of bits of potato in sucrose solutions of varying molarities - where you determine the isotonic point and therefore the corresponding w. potential of the potato cells.

Just wondering if anyone could tell me what the final graph shape is "supposed" to be, I vaguely remember someone in my class last year having a sigmoid shape - which is similar to the one I'm looking at now actually. But I'm not really all that sure why the ends of the graph would both tail off. I mean, I guess it could be explained in terms of being at a maximum turgidity and not being able to gain any more water for the ones which gain mass...but the ones which lose mass?

I don't know if anyone has done this with OCR, but the criteria aren't very clear on any stats/error bars which may be needed either, so I'd be grateful if anyone knew whether they were necessary.

Cheers.

Reply 1

Kyalimers

21

Depends on the axes.... It will be acurve that tails off and straightens up horizontally...Tell me the axes and i'll help some more.

As for your q: the potato cylinders only hold so much water!!! i.e. take a buvket and empty ait and there is nothing left to empty out!!! It's similar with the potatoes. Also the water coming out of the potato may equilibrate the conc. of the solutions inside and outside of the potato therefore tailing the graph off.

Reply 2

purple-girl

17

Right, it is basicaly supposed to be an inverse sigmoid shape as you put it. It levels off at the top when the sucrose concentration is low as the plant cells are fully turgid and can't take in any more water. At the bottom when the sucrose conc. is high then the cells are plasmolysed and so cannot lose any more water.

How do you mean stats/error bars? We were told that anomalies are points that are more than 10% away from the line of best fit, and that we had to mention things that could cause anomalies, if that's the kinda thing you mean. I'm on AQA by the way.

EDIT: Nice analogy there Sohan :tongue:

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Reply 3

Kyalimers

21

purple-girl

Right, it is basicaly supposed to be an inverse sigmoid shape as you put it. It levels off at the top when the sucrose concentration is low as the plant cells are fully turgid and can't take in any more water. At the bottom when the sucrose conc. is high then the cells are plasmolysed and so cannot lose any more water.

How do you mean stats/error bars? We were told that anomalies are points that are more than 10% away from the line of best fit, and that we had to mention things that could cause anomalies, if that's the kinda thing you mean. I'm on AQA by the way.

EDIT: Nice analogy there Sohan :tongue:

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Hey Kat: long time no see eh!! Thnx though. How did your Biology go???

Reply 4

emilY?

OP

1

Thanks folks, yeah went for the inverse sigmoid shape with the maximum turgidity and then plasmolysis!

Good stuff, I hadn't read your replies by then so at least I know now I didn't destroy her coursework!

Osmosis. Potatoes. You know the one...

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